Re: [PATCH hmm 00/15] Consolidate the mmu notifier interval_tree and locking

[Jerome Glisse <jglisse-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, Oct 23, 2019 at 11:32:16AM +0200, Christian König wrote:
> Am 23.10.19 um 11:08 schrieb Daniel Vetter:
> > On Tue, Oct 22, 2019 at 03:01:13PM +0000, Jason Gunthorpe wrote:
> > > On Tue, Oct 22, 2019 at 09:57:35AM +0200, Daniel Vetter wrote:
> > > 
> > > > > The unusual bit in all of this is using a lock's critical region to
> > > > > 'protect' data for read, but updating that same data before the lock's
> > > > > critical secion. ie relying on the unlock barrier to 'release' program
> > > > > ordered stores done before the lock's own critical region, and the
> > > > > lock side barrier to 'acquire' those stores.
> > > > I think this unusual use of locks as barriers for other unlocked accesses
> > > > deserves comments even more than just normal barriers. Can you pls add
> > > > them? I think the design seeems sound ...
> > > > 
> > > > Also the comment on the driver's lock hopefully prevents driver
> > > > maintainers from moving the driver_lock around in a way that would very
> > > > subtle break the scheme, so I think having the acquire barrier commented
> > > > in each place would be really good.
> > > There is already a lot of documentation, I think it would be helpful
> > > if you could suggest some specific places where you think an addition
> > > would help? I think the perspective of someone less familiar with this
> > > design would really improve the documentation
> > Hm I just meant the usual recommendation that "barriers must have comments
> > explaining what they order, and where the other side of the barrier is".
> > Using unlock/lock as a barrier imo just makes that an even better idea.
> > Usually what I do is something like "we need to order $this against $that
> > below, and the other side of this barrier is in function()." With maybe a
> > bit more if it's not obvious how things go wrong if the orderin is broken.
> > 
> > Ofc seqlock.h itself skimps on that rule and doesn't bother explaining its
> > barriers :-/
> > 
> > > I've been tempted to force the driver to store the seq number directly
> > > under the driver lock - this makes the scheme much clearer, ie
> > > something like this:
> > > 
> > > diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.c b/drivers/gpu/drm/nouveau/nouveau_svm.c
> > > index 712c99918551bc..738fa670dcfb19 100644
> > > --- a/drivers/gpu/drm/nouveau/nouveau_svm.c
> > > +++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
> > > @@ -488,7 +488,8 @@ struct svm_notifier {
> > >   };
> > >   static bool nouveau_svm_range_invalidate(struct mmu_range_notifier *mrn,
> > > -                                        const struct mmu_notifier_range *range)
> > > +                                        const struct mmu_notifier_range *range,
> > > +                                        unsigned long seq)
> > >   {
> > >          struct svm_notifier *sn =
> > >                  container_of(mrn, struct svm_notifier, notifier);
> > > @@ -504,6 +505,7 @@ static bool nouveau_svm_range_invalidate(struct mmu_range_notifier *mrn,
> > >                  mutex_lock(&sn->svmm->mutex);
> > >          else if (!mutex_trylock(&sn->svmm->mutex))
> > >                  return false;
> > > +       mmu_range_notifier_update_seq(mrn, seq);
> > >          mutex_unlock(&sn->svmm->mutex);
> > >          return true;
> > >   }
> > > 
> > > 
> > > At the cost of making the driver a bit more complex, what do you
> > > think?
> > Hm, spinning this further ... could we initialize the mmu range notifier
> > with a pointer to the driver lock, so that we could put a
> > lockdep_assert_held into mmu_range_notifier_update_seq? I think that would
> > make this scheme substantially more driver-hacker proof :-)
> 
> Going another step further.... what hinders us to put the lock into the mmu
> range notifier itself and have _lock()/_unlock() helpers?
> 
> I mean having the lock in the driver only makes sense when the driver would
> be using the same lock for multiple things, e.g. multiple MMU range
> notifiers under the same lock. But I really don't see that use case here.

I actualy do, nouveau use one lock to protect the page table and that's the
lock that matter. You can have multiple range for a single page table, idea
being only a sub-set of the process address space is ever accessed by the
GPU and those it is better to focus on this sub-set and track invalidation in
a finer grain.

Cheers,
Jérôme

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